Study of multi-twisted-tube gas cooler for CO2 heat pump water heaters

Multi-twisted-tube type heat exchangers applied in a transcritical CO2 heat pump as gas coolers were investigated. The influence of different numbers of inner tubes on the heat transfer and the pressure drop was discussed by means of experiments and theoretical analysis. The performances of two gas coolers with three and four inner tubes were measured and compared. It is found that heat transfer characteristics of the gas cooler with four inner tubes is better than that with three inner tubes. Based on the theoretical analysis, outlet temperature of water increases with increasing the number of inner tubes at the beginning and the increasing tendency becomes gentle when the number of inner tubes is more than four. But the pressure drop of gas cooler rises greatly with the increment of the number of inner tubes. It is implied from the analysis that the coefficient of performance of CO2 heat pump is not optimum when the outlet temperature of water becomes high by increasing mass flow rate of CO2 and decreasing the mass flow rate of water. In order to get both high COP and high outlet temperature of water, the water side heat transfer coefficient of the gas cooler needs improving.